Stable iodized calcium phosphate



Patented Mar. 25, 1947 STABLE IODIZED CALCIUM PHOSPHATE No Drawing.Application September 12, 1941, Serial No. 410,630

4. Claims. 1

This invention relates to a stable iodized calcium phosphate and amethod of preparing the same.

It is the common practice in the United States to add potassium iodideto table salt and certain other granular food chemicals in order toprovide a source of iodine in the commodity. Potassium iodide, however,decomposes slowly, liberating iodine which frequently causes anundesirable discoloration of the product.

Ordinary commercial tricalcium phosphate- 3Cas (PO4)2.Ca(OI-I)2-issometimes used for conditioning table salt or other granular chemicalmaterials. The addition of the ordinary tricalcium phosphate, however,in combination with potassium iodide, particularly in the presence ofcommon salt, does not stabilize potassium iodide. On the contrary thesalt will turn to a yellowish brown color within a few days underordinary atmospheric storage conditions when as little as 0.02%potassium iodide is present.

It has now been discovered, however, that a basic tricalcium phosphatemay be prepared which is inherently more alkaline than the normalcommercial product, and which may be intimately mixed with potassiumiodide even in the presence of salt and other food chemicals withoutgeneration of iodine. The basic tricalcium phosphate may be prepared byreacting a dilute solution of phosphoric acid with a weak milk of limeslurry, employing a sufiicient excess of the milk of lime to give thereaction mixture a pH value (based upon a 1% slurry) above 3.0 andpreferably about 10.0. Under these conditions the precipitated basictricalcium phosphate will have a composition approachingCaz(PO4)2.Ca(OI-I)2. In other words, the ratio of OH radical to P205approaches approximately 2 to 1. In ordinary hydroxy tricalciumphosphate the ratio is 2 to 3. The basic tricalcium phosphate is asubstance having sufficient excess OH groups to provide a pH value above8.0 and approaching the compound CA3 (PO4)2.C2L(OH)2.

The OH groups may be supplied in part at least by caustic soda. Forexample, phosphoric acid and milk of lime may be reacted in proportionsto give tricalcium phosphate having a pH value of about 7.0, after whichsuflicient caustic soda is added to give the mixture a pH value above8.0.

In either case the precipitated basic tricalcium phosphate is filteredofi, dried, and milled to pass through a 200 mesh or smaller screen.

The product is then thoroughly mixed with a small proportion, generallyless than of 2 finely milled potassium iodide. position containsapproximately 97% to 99% of the basic tricalcium phosphate, and 3% to 1%A preferred comof potassium iodide. This composition is preferably foruse with ordinary table salt since it may be employed in amountssuflicient not only to condition the salt to give it free-flowingqualities but to introduce at the same time a suitable amount of iodide.Normally the ratio of the basic tricalcium phosphate-potassium iodidemixture to the salt is about 0.5% to 1%. Generally not over 3%tricalcium phosphate is required to make the product free flowing. Theratio of potassium iodide to tricalcium phosphate is adjusted so thatwhen enough tricalcium phosphate is used to make the product freeflowing, the proportion of iodide to salt will be proper.

For example, a granular table salt was prepared by including 0.5% ofbasic tricalcium phosphate-potassium iodide mixture in which the ratioof OH groups to P205 groups was approximately 5 to 3, and the proportionof potassium iodide 2% of the complex. After a storage period of threemonths, the salt mixture showed no discoloration or other evidence'ofiodide decomposition.

On the other hand, a mixture prepared with the same salt using 0.5% ofordinary tricalcium phosphate having an OH-tO-PzOs ratio ofapproximately 2 to 3, and a pH of approximately '7, and containing 2% ofpotassium iodide, developed a distinctly brownish color after three daysand emitted a strong iodine odor.

In order to effect its stabilizing action, basic tricalcium phosphatemust be intimately admixed with the potassium iodide. Whether a chemicalreaction occurs between them has not yet been ascertained.

The new product has improved properties of stabilization as comparedwith a normal tricalcium phosphate to which sodium bicarbonate or otheralkaline material has been added together with potassium iodide. Whilesalt so treated with separately alkalized tricalcium phosphate andpotassium iodide keeps better than salt treated with ordinary tricalciumphosphate, the product compares unfavorably with that of the presentinvention.

The basic tricalcium phosphate may be added to salt and potassiumiodide, then mixed with the compound, or potassium iodide may be addedto the salt and the basic tricalcium phosphate added. However, thenecessary intimate contact is best secured by mixing the basictricalcium as used in the claims, however, refers to the chemicalequivalents of that product.

The term compound alkaline tricalcium phosphate as used herein denotestricalcium phosphate compounded as indicated herein in which the excessalkali is supplied by a wet process at the time of manufacture of thecomposition as distinguished from a mechanical mixture of the excessalkaline material with tricalcium phosphate.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom.

What I claim as new, and desire to secure by Letters Patent, is:

1. A composition comprising a stable mixture of compound alkalinetricalcium phosphate and a minor amount of potassium iodide, saidalkaline tricalcium phosphate having a (32.0 to P205 molecular ratio ofsubstantially above the normal 3.33 to 1 and a pH value of 10.0 in 1%aqueous suspension.

2. A composition comprising a, stable mixture of compound alkalinetricalcium phosphate and a minor amount of potassium iodide, saidalkaline tricalcium phosphate having a CaO to P205 substantially abovethe normal molecular ratio of 3.33 to 1 and a pH value of at least 10 in1% aqueous suspension.

3. A composition as set forth in claim 2, in which the amount ofpotassium iodide is at lea-st 0.5% and less than 10% of the composition.

4. A composition as set forth in claim 2, in which the amount ofpotassium iodide is at least 0.5% and not substantially more than 3% ofthe composition.

HOWARD ADLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Industrial & EngineeringChemistry, Feb. 1933, pages 142 through 147 by Moss, Schilb and Warning.

